Sea Level controls on Agulhas Leakage Salinity and the Atlantic Overturning Circulation

Sophie Nuber (  sophie.nuber@hotmail.de ) National Taiwan University James Rae University of St Andrews https://orcid.org/0000-0003-3904-2526 Morten Andersen School of Earth and Ocean Sciences, Cardiff University Xu Zhang State Key Laboratory of Tibetan Plateau Earth System, Resources and Environment (TPESRE), Institute of Tibetan Plateau Research, Chinese Academy of Sciences https://orcid.org/0000-0003-1833-9689 Bas de Boer Vrije Universiteit Amsterdam Matthew Dumont University of St Andrews Yuchen Sun https://orcid.org/0000-0002-2449-8718 Huw Mithan National Taiwan University Ian Hall School of Earth and Ocean Sciences Sciences https://orcid.org/0000-0001-6960-1419 Stephen Barker Cardiff University

subtropical gyre with comparable surface salinities. However, this is not the case. Modern Indian Ocean surface conditions are notably fresher than comparable latitudes in the Atlantic and Paci c. This is due to the in ow of monsoon-derived and Paci c-origin low salinity surface and thermocline waters from the Indonesian Through ow (ITF) (Figure 1). In ow occurs via the Timor, Lombok, and Ombai Straits (Sprintall et al., 2009), as well as the Bay of Bengal (Sengupta et al., 2006), and is transported across the tropical Indian Ocean via the South Equatorial Current (SEC) (Talley & Sprintall, 2005;Sengupta et al., 2006;Gordon et al., 1997). Surface water salinity does increase within the subtropical western Indian Ocean (Talley & Sprintall, 2005), but the salt is then partially exported through an active Agulhas Leakage (Durgadoo et al., 2017). The addition of Indian Ocean salinity to Atlantic surface waters via the Agulhas Leakage has been proposed as a mechanism to in uence global ocean circulation by enhancing the density potential at North Atlantic deep-water convection sites (Beal et al., 2011;Biastoch et al., 2009). It is therefore possible that changes in Indian Ocean surface salinity could directly impact global ocean circulation.
Here, we present coupled SST (from Mg/Ca in the planktonic foraminifer Globigerinoides ruber) and relative salinity reconstructions (see Methods 1) from western Indian Ocean IODP Site U1476 located in the northern entrance of the Mozambique Channel (15°49.25'S; 41°46.12'E; 2166m). This site is strongly in uenced by the westward owing SEC (Durgadoo et al., 2017), and thus tracks the hydrographic conditions of the western Indian Ocean source waters that feed the Agulhas leakage. Our 1.2 million year (Ma)-long record provides the rst evidence for changes in tropical western Indian Ocean hydrography beyond the glacial maximum (LGM). We examine the controls and impacts of these changes to assess the role of Agulhas Leakage salt content in the global overturning circulation and climate.
Our reconstructions show that western Indian Ocean surface salinity and temperature structures were signi cantly different from modern during Pleistocene glacial stages. Figure 2 shows that western SEC temperature initially cools during glacial inception from its interglacial high. However during the middle and latter phases of glacial cycles, this cooling trend reverses and surface waters begin to warm.
In the absence of other in uences, this warming would be expected to cause a decrease in planktic δ 18 O.
However, we nd planktic δ 18 O continues to increase, suggesting an increase in δ 18 O sw and hence surface salinity. Global growth in ice volume can only account for 50% of the glacial-interglacial difference in δ 18 O sw across the last 1.2Ma (see supplementary Figure S1). As such, our data indicate a regional increase in sea surface salinity (as well as temperature) during glacial periods. (see Methods 5). The model output gives spatial information on sea level height changes through time and allows us to assess the timing of land exposure and ooding due to sea level dynamics. Our model results show that the greatest changes in sea-to-land surface area across the Indonesian archipelago occur when RSL is between -2m to -8m, and -45m to -55m ( Figure S4). The shallower of these intervals appears to be linked to the initial exposure of shallow marine geomorphological landforms, such as shallow submarine island channels, estuaries, terraces, shore platforms, sand banks, and coral reefs. Its impact on regional circulation and Indian Ocean surface salinity is therefore small. The deeper interval is directly linked to the abrupt exposure of the Java Straits and North Australian continental shelves. This resurfacing of land in the Indonesian Archipelago will have important implications for the out ow dynamics of the ITF. Indeed, we nd a close correspondence between RSL and ITF out ow strength, as reconstructed from δ 13 C benthic in the Lombok Strait (Holbourn et al. 2011) (Figure 3g, h). This suggests that lowering global sea levels causes a reduction in the ITF out ow due to abrupt land surfacing in the Indonesian Archipelago. The Java Straits and North Australian shelves have both been previously hypothesised to in uence regional circulation and surface water hydrography. The exposure of North Australian shelves may reduce the out ow pro le area, and therefore the out ow volume of the ITF  Figure S5), providing evidence that the western Indian Ocean evolved similarly to the rest of the basin. In sum, we suggest that the glacial salini cation process in the surface Indian Ocean is a result of the abrupt reorganisation of land masses in the Indonesian archipelago driving a reduction in the ITF, and therefore a reduction in freshwater entering the SEC in the Indian Ocean. The occurrence of these events during times of generally weak Agulhas Leakage and high evaporation fostered the recirculation of water masses in the subtropical and tropical Indian Ocean gyre, and elevated salinity and temperature in surface waters at times of global glaciation.
The high salinity conditions in the glacial Indian Ocean can be traced along the AL system pathway and would therefore have in uenced AL waters after AL resumption. Published AL reconstructions show a resumption of the AL volume ow at the onset of deglaciations (Figure 3i), and these leakage waters have been shown to be particularly salty (Marino et al., 2013). According to our results, the high salinity observed after AL resumption is likely sourced from the glacial Indian Ocean, whose salini cation is highest just before deglaciation (Figure 3c). Climate model results under future climate change conditions suggest that a saltier AL directly impacts the global ocean circulation (Biastoch et al., 2009).
However, little is known about the impact of a salty AL on the global overturning circulation during the onset of deglaciation. We therefore tested the in uence of a salty AL on the overturning circulation using a fully-coupled atmosphere-ocean global circulation model COSMOS (Zhang et al., 2013) for a deglacial freshwater event scenario with LGM parameters (see Methods 6). We rst performed a classic North Atlantic hosing experiment (LGM_015) applying 0.15Sv into the so-called Ruddiman Belt for 500 years to generate a weakened AMOC background under LGM conditions and thus mimic an early deglacial Heinrich stadial (see Zhang et al., 2013). To simulate an increased AL supplying saltier Indian Ocean water masses to the Atlantic, we then conducted two freshwater extraction experiments based on LGM_0.15 (persistently hosing in the North Atlantic) by additionally imposing constant high evaporation uxes over the Agulhas plateau, equivalent to 0.05Sv (LGM_015 SA005) and 0.15Sv (LGM_015 SA01) saltwater input. After inducing salini cation in the Agulhas plateau area, we see a complete AMOC recovery from around 7Sv to a peak of ~25Sv in LGM_015 SA005 and ~30Sv in LGM_015 SA01, with the onset of AMOC recovery occurring after ~400 years and ~250 years respectively (Figure 4). This suggests that a highly saline AL can have a direct impact on a Heinrich weakened AMOC, and can lead to an AMOC recovery even under a persistent freshwater input in the North Atlantic (which might be expected during deglaciation). Our results further suggest that the amount of salt available in the AL has an effect on the response time of the AMOC, as well as the recovery strength, with higher salinities leading to faster response times and stronger recovery. This underlines that the glacial salini cation process in the tropical and subtropical Indian surface Ocean can have a direct in uence not only on global overturning circulation, but also on the shape of deglaciations, depending on the amount of salt harvested throughout the glacial, and the speed of release through the AL.    Simulated global responses to increased Agulhas Leakage salinity during North Atlantic freshwater perturbation events, representating deglacial onset conditions. a) AMOC index [Sv] for experiments with strong (LGM015_SA01, blue line) and weak (LGM015_SA005, red line) increase in Agulhas Leakage salinity, forced respectively by 0.1 and 0.05Sv evaporation anomalies over the Agulhas Plateau, compared to an experiment with no evaporation forcing shown in black (LGM015). The AMOC index is de ned as the maximum of Atlantic meridional transport stream function north of 45°N at a water depth between 1000m and 2500m. b) and c) are responses of mean annual surface air temperature (°C) and precipitation (mm/mon) to an AMOC recovery state in LGM015_SA005 (i.e. anomalies between LGM015_SA005 and LGM015 for intervals indicated by red bar in panel a)).